1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a printing medium.
2. Description of the Related Art
The performance of various types of image forming apparatuses (such as color printers and color copiers) has been improved in recent years and images of higher quality can be formed. On the other hand, there is also the possibility that a malicious individual will utilize such an image forming apparatus that is capable of forming high-quality images. The reason is that such an image forming apparatus is capable of faithfully reproducing paper currencies, banknotes and securities, etc. This problem of counterfeiting is becoming more worrisome as the performance of image forming apparatuses improves.
The specification of Japanese Patent Laid-Open No. 10-76735 proposes a technique whereby an additional signal indicating an ID that represents the manufacturer of the image forming apparatus or the serial number of the image forming apparatus is added to a color signal as a measure for suppressing counterfeits and forgeries. Further, the specification of Japanese Patent Laid-Open No. 2000-253229 proposes an invention for extracting an additional signal from an input image and specifying the apparatus that formed the image based upon the additional signal extracted.
A dot pattern serving as the additional signal usually is added to the entire image. Further, the dot pattern is added only to the yellow plane in order to make it difficult for the pattern to be identified by the human eye. However, a yellow dot pattern added to an OHT (overhead transparency) sheet can readily be identified by the human eye. This is undesirable in terms of image quality. Further, since an OHT sheet has a paper quality much different from that of paper currency, banknotes and securities, a forgery can be discerned easily. For these reasons, there is little need to add a dot pattern to an OHT sheet. Japanese Patent Laid-Open No. 10-76735 proposes an invention in which a tracking pattern is not added if a printing medium designated by the user is an OHT sheet.